MFC r287706 (by delphij):

MFC r287706 (by delphij):
6214 zpools going south

In r286570 (MFV of r277426) an unprotected write to b_flags to
set the compression mode was introduced.  This would open a race
window where data is partially decompressed, modified, checksummed
and written to the pool, resulting in pool corruption due to the
partial decompression.

Prevent this by reintroducing b_compress

illumos/illumos-gate@d4cd038c92c36fd0ae35945831a8fc2975b5272c

Illumos issues:

    6214 zpools going south
    https://www.illumos.org/issues/6214

git-svn-id: svn://svn.freebsd.org/base/stable/10@288596 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f